I don't want to actually install it permanently.

I'll do what I did last time and remove the bottom half of the iMac, flip the unit upside-down, and just temporarily set the CD drive on top of it while it installs the OS. Once it is installed I'll put it all back together without the CD drive.

I tried this once with a fullsize CDROM from a tower and a molex splitter to power it, but that didn't work for some reason.